Plenty of apps and devices encourage you to work out more, more, more! But the Ithlete Finger Sensor ($69.99, direct) and its companion app Myithlete (free) might say, "Not so fast." The Ithlete system measures heart rate variability to tell you if you need a day of rest, should stick to light cardio exercise, or have a green light to work out as hard as you want.

Heart rate variability (HRV) is a comparative measurement of the time that elapses in between your heartbeats. The higher the score, the more variations in those intervals, so higher is better. Healthy adults have an Ithlete score above 55. My readings tended to be in the 90s. The highest score anyone at HRV Fit (the company behind Ithlete) has seen is 116. Because HRV is a little complex to understand, Ithlete's app, Myithlete, tries to make it simple. After each reading (described in detail below) the app gives you a score and a color recommendation. Green is a free pass to work out hard. Orange means take it easy with cardio only for today. And red indicates you need a day off.

You can add more detail to each reading by telling the app how fatigued you feel, whether you slept well last night, if you have any muscle soreness, your mood, and more. Ithlete keeps track of all those extra data points and plots them over a chart to help you detect patterns, such as whether a very good HRV reading typically follows a day when you reported eating especially well.

How to Use IthleteTo use Ithlete, you plug the Finger Sensor into a smartphone's or iPad's headphone jack, and crank the volume as high as it can go. It's compatible with iPhone 3GS and later; third-generation iPod touch and later; iPad and iPad mini; Samsung Galaxy S3 and S4; and the HTC One. Then, you launch the app and place your index or middle finger in the finger cup. You can feel a raised dot inside, an infra-red sensor. The app and device detect your pulse and display it at the bottom left corner when they have a good signal. When the app is ready to go, a start button appears.

Pressing the start button begins a 55-second HRV reading, during which time the app tells you to breathe in through your nose and out through your mouth in time with a slowly pulsating graphic. If the sensor loses your heart rate, the app aborts the session and asks you to start over again, which happened to me maybe one out of every six or seven times.

Information in the app (hidden in the settings) actually warns that "people with poor circulation in their fingers, low blood pressure, or who are wearing thick nail varnish may experience problems and are advised to choose another sensor type." Children can't use the sensor either because their fingers are too small to fit snugly in the cup.

Readings and RecommendationsYou need to use the app for about a week for it to develop a baseline understanding of your heart health and make adequate recommendations. And you're supposed to take a reading immediately upon waking in the morning, before you eat or drink anything.

After a few days of collecting data, I didn't find any perceptible patterns that might steer me toward or away from exercising—although one caveat is I'm not an athlete in training, which is the target market for Ithlete. I did notice, however, that on the rare occasions I had a bad night's sleep, my HRV was noticeably lower (in the 80s as opposed to mid-90s), and I was more likely to see red or orange on those days, too.

For several days in a row, I took two readings back to back, using an iPhone for one and an iPad mini for the other. Usually the numerical results were close, and sometimes they matched exactly. The color recommendations weren't always the same, however. For example, on Jan. 21, I got 84 and 88, and an orange recommendation for both, whereas on Jan. 29, I got 97 with red, and 95 with green.

The app also records resting heart rate (RHR) for each session, and when I compared my two data sets, these were never off from one another by more than two integers, giving me some assurance that the app was collecting at least some data reliably.

You can export your data to a CSV table that you can either email to yourself or save to Dropbox. The app also has an included alarm to remind you to take your reading first thing every morning.

Rest or Go? If you are want a clear recommendation on when to train and when to rest, Ithlete Finger Sensor and Myithlete app can give you some advice based on biometric feedback. It's an innovative yet simple solution to an age-old problem. While it's hard to know how accurate the data and recommendations are, the device and app are extremely simple to use and reasonably priced. The alternative? Listen to your coach, health care provider, and your own body.

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Jill Duffy is a contributing editor, specializing in productivity apps and software, as well as technologies for health and fitness. She writes the weekly Get Organized column, with tips on how to lead a better digital life. Her first book, Get Organized: How to Clean Up Your Messy Digital Life is available for Kindle, iPad, and other digital formats. She is also the creator and author of ProductivityReport.org.
Before joining PCMag.com, she was senior editor at the Association for Computing Machinery, a non-profit membership organization for...
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